Thanks for all the comments and observations.
Multinationals - I agree that if multinationals were allowed join or fund the ITA, it would be counterproductive. Membership would be confined to individuals and, maybe SMEs. Britain's Taxpayers Alliance, for example, does not accept multinational money. Funding could come from individual membership fees and donations limited to, say, 1000 euro/person/year. There may even be some spare NGO money floating around that the ITA could apply for.
Income tax or VAT - I don't think the income taxpayer v the VAT payers is useful. The power of the ITA would come from the fact they represent taxpayers, i.e. the majority of voters, who want their taxes collected fairly, their allowances and exemptions fairly and quickly administered, and their taxes used efficiently.
Too broad a church? Although its true that the category 'taxpayer' is so amorphous it could be difficult to rally around a single cause, the solution to this would be to keep the ITA narrowly focused. For example, the ITA would never wade into the debate of whether using taxes for immigrant housing is a good use of resources, as this would be far too divisive. But I think all taxpayers would agree that all government departments should report spending is a consistent format for easy analysis (a point recently highlighted by David McWilliams in the IT) and there are many other issues that are politically neutral and that all taxpayers would agree with, regardless of their politics. Those would be the sort of issues the ITA could address.